U.S. patent number 6,921,830 [Application Number 10/475,219] was granted by the patent office on 2005-07-26 for method for purifying an organic solvent for the purposes of absorption of maleic acid anhydride.
Invention is credited to Gerd Kaibel, Ralf-Thomas Rahn, Alexander Weck.
United States Patent |
6,921,830 |
Rahn , et al. |
July 26, 2005 |
Method for purifying an organic solvent for the purposes of
absorption of maleic acid anhydride
Abstract
In a process for purifying an organic solvent for the absorption
of maleic anhydride from a gaseous mixture, in which the maleic
anhydride is separated off from the solvent, a substream is taken
from the solvent stream, this substream is distilled and returned
to the solvent circuit, the proposed improvement comprises
separating off a low-boiling fraction and a high-boiling fraction
from the fraction consisting essentially of purified solvent and
feeding the fraction consisting essentially of purified solvent
back into the solvent stream.
Inventors: |
Rahn; Ralf-Thomas (68167
Mannheim, DE), Weck; Alexander (67251 Freinsheim,
DE), Kaibel; Gerd (68623 Lampertheim, DE) |
Family
ID: |
7682335 |
Appl.
No.: |
10/475,219 |
Filed: |
October 20, 2003 |
PCT
Filed: |
April 22, 2002 |
PCT No.: |
PCT/EP02/04414 |
371(c)(1),(2),(4) Date: |
October 20, 2003 |
PCT
Pub. No.: |
WO02/08583 |
PCT
Pub. Date: |
October 31, 2002 |
Foreign Application Priority Data
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Apr 23, 2001 [DE] |
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101 19 737 |
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Current U.S.
Class: |
549/262; 549/258;
549/259 |
Current CPC
Class: |
C07C
51/573 (20130101); C07C 51/215 (20130101); C07C
51/215 (20130101); C07C 57/145 (20130101); C07C
51/573 (20130101); C07C 57/145 (20130101) |
Current International
Class: |
C07C
51/16 (20060101); C07C 51/573 (20060101); C07C
51/54 (20060101); C07C 51/215 (20060101); C07D
307/36 () |
Field of
Search: |
;549/262,258,259 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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459 543 |
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Dec 1991 |
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EP |
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897 905 |
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Feb 1999 |
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EP |
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96/29323 |
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Sep 1996 |
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WO |
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Primary Examiner: Dentz; Bernard
Parent Case Text
This application is a 371 of PCT/EP02/04414 filed Apr. 22, 2002.
Claims
We claim:
1. A process for purifying an organic solvent for the absorption of
maleic anhydride from a gaseous mixture, where the maleic anhydride
is separated from the solvent, a substream is taken from the
solvent stream and this substream is distilled and returned to the
solvent circuit, wherein a low-boiling fraction and a high-boiling
fraction are separated off from the fraction consisting essentially
of purified solvent and the fraction consisting essentially of
purified solvent is fed back into the solvent stream.
2. A process as claimed in claim 1, wherein the distillation is
carried out at a pressure of from 0.1 to 100 mbar.
3. A process as claimed in claim 1, wherein the low-boiling
fraction is taken off continuously at the top of a distillation
column, the high-boiling fraction is taken off continuously from
the bottom of the column and the purified solvent is taken off
continuously in gaseous or liquid form via the side offtake of the
column.
4. A process as claimed in claim 1, wherein the low-boiling
fraction and the fraction consisting essentially of purified
solvent are distilled off continuously from the high-boiling
fraction, and the low-boiling fraction is separated off from the
distillate via the top and the fraction consisting essentially of
purified solvent is separated off at the bottom.
5. A process as claimed in claim 1 which is carried out batchwise
or semicontinuously.
6. A process as claimed in claim 1, wherein the organic solvent
used has a boiling point which, at atmospheric pressure, is at
least 30.degree. Kelvin above the boiling point of the low-boiling
fraction.
7. A process as claimed in claim 1, wherein the distillation is
carried out using a dividing wall column.
8. A process as claimed in claim 1, wherein the distillation is
carried out at a pressure of from 1 to 20 mbar.
Description
The present invention relates to a process for purifying an organic
solvent for the absorption of maleic anhydride from a gaseous
mixture.
In the industrial preparation of maleic anhydride (MA), which is
nowadays carried out predominantly by oxidation of n-butane by
means of air or oxygen in the gas phase, MA is separated from the
offgas by absorption. The MA can be absorbed into water or, as is
generally considered more favorable, into an organic solvent.
Suitable solvents are described, for example, in WO 96/29323.
After absorption has occurred, the MA-containing solution is worked
up further. To prepare crude MA, the absorption step is followed by
single-stage or multistage stripping, with the crude MA being
obtained at the top of the column. This stripping is carried out
either under reduced pressure, preferably at from 50 to 100 mbar,
or by means of nitrogen, hydrogen or a mixture of nitrogen and
hydrogen as stripping gas. The absorption medium which has been
largely freed of MA is obtained at the bottom of the column and is
recirculated to the absorption. However, an increase in the
concentration of maleic acid and fumaric acid is observed with
increasing number of recirculations. Furthermore, the formation of
further acidic constituents, e.g. monoalkyl phthalates and phthalic
anhydride, is also observed. In addition, tar-like high boilers are
also formed. WO 96/29323 describes the removal of the tar-like
components and high boilers by extraction with water. In the tests
reported, a degree of removal of a maximum of 50% is achieved, with
the formation of emulsions being observed, especially at the very
high degrees of removal, resulting in problems in phase
separation.
EP-A-897 905 describes the extraction of acidic compounds by means
of aqueouos alkaline solutions in the preparation of MA. According
to this method, a gaseous reaction mixture comprising maleic
anhydride is brought into contact with an organic solvent, at least
part of the maleic anhydride is separated off from the organic
solvent, part of the organic solvent remaining after the maleic
anhydride has been separated off is scrubbed with an aqueous alkali
solution, for example sodium hydroxide or ammonia solution, and the
scrubbed organic solvent and the remaining organic solvent are
returned to the absorption of the maleic anhydride from the gaseous
reaction mixture. Compared to extraction using pure water,
extraction with alkali is said to lead to better results. The
document says nothing about problems in the phase separation in the
simultaneous presence of high boilers.
A disadvantage of the processes described in the above documents is
the relatively low degree of removal, in particular in the case of
tar-like polymers. Difficulties in phase separation when the
extracted proportion of high boilers approaches 50% are likewise
disadvantageous. Another disadvantage is the fact that the aqueous
phase has to be disposed of. Discharge into the wastewater system
may be problematical, since the organic material in the aqueous
phase has to have a prescribed degradability. Incineration suffers
from drawbacks particularly when alkali metal salts are present in
the aqueous phase.
Apart from the disposal problems associated with the aqueous phase,
the water content of the organic phase likewise constitutes a
problem. In general, a higher water content in the solvent in the
absorption of MA leads to increased formation of maleic acid, so
that, in addition to the extraction stage which generally consists
of a cascade of mixer-settlers, there may also be a need for a
vaporization step to separate off residual water.
U.S. Pat. No. 4,118,403 describes a process for purifying an
organic solvent for the absorption of maleic anhydride from a
gaseous mixture, in which the maleic anhydride is separated from
the solvent, a substream is taken from the solvent stream, this
substream if distilled and is returned to the solvent circuit. As
can be seen from the figure in the document, the entire solvent
stream after the maleic anhydride has been separated off in the
stripping column 16 is cooled and filtered in a cooling/filtration
apparatus 41. A substream is subsequently discharged via the
distillation apparatus 42 and is distilled to separate off the high
boilers. The distillate is then fed back into the solvent
circuit.
A disadvantage of this known process is that maleic acid and
fumaric acid accumulate in the solvent circuit and the acid content
thus increases. The presence of acid promotes the reaction of
maleic anhydride with water to form maleic acid and the yield of MA
is thus decreased. These drawbacks have been confirmed in
laboratory tests carried out by us.
It is an object of the present invention to avoid the disadvantages
known from the prior art and in particular to avoid accumulation of
acidic components in the solvent stream.
We have found that this object is achieved by a process having the
features of claim 1, in which a low-boiling fraction and a
high-boiling fraction are separated off from the fraction
consisting essentially of purified solvent and the fraction
consisting essentially of purified solvent is fed back into the
solvent stream.
The present invention makes it possible to separate off not only
the high boilers but also the acidic compounds maleic acid, fumaric
acid, phthalic acid, etc., before the solvent is returned to the
absorption of gaseous MA. This is achieved with a small outlay in
terms of apparatus and without use of extraneous substances. The
low-boiling fraction separated off in the process of the present
invention comprises as significant components maleic acid, fumaric
acid, phthalic acid and small amounts of maleic anhydride and the
solvent used. The high-boiling fraction comprises as significant
components tar-like products and small amounts of the solvent used.
In the fraction consisting essentially of purified solvent, the
solvent is present in a purity which enables this fraction to be
returned directly to the solvent circuit.
The process of the present invention can be carried out
continuously, semicontinuously or batchwise.
In a first, continuous embodiment of the process of the present
invention, a substream of the contaminated solvent is fed to a
distillation column. Here, the low-boiling acidic components maleic
anhydride, maleic acid, fumaric acid and phthalic anhydride go over
at the top, the tar-like components are taken off at the bottom and
the largely by-product-free solvent, for example dibutyl phthalate,
is obtained via the side offtake. The distillation is
advantageously carried out under reduced pressure, preferably at a
pressure of from 0.1 to 100 mbar, more preferably at a pressure of
from 1 to 20 mbar, particularly preferably at a pressure of from 2
to 15 mbar. When using a conventional column, the purified solvent,
e.g. dibutyl phthalate, is generally taken off in the stripping
section. The stream taken off at the side offtake will therefore be
gaseous.
The separation generally requires from about 1 to 60, in particular
from about 10 to 50, theoretical plates, preferably from 15 to 40,
particularly preferably from 15 to 30, theoretical plates. A
definition of theoretical plates may be found, for example, in
Klaus Sattler, Thermische Trennverfahren, p. 5, right-hand column,
paragraph 2, ISBN 3-527-26727-1. The side offtake is advantageously
located from 1 to 5 theoretical plates above the bottom, preferably
from 1 to 4 theoretical plates above the bottom.
In one variant, the distillation can be carried out in a dividing
wall column as described in U.S. Pat. No. 2,471,134, in EP-A-0 122
367 or by G. Kaibel, Chem. Eng. Technol. Vol. 10, 1987, pp. 92-98.
In this case, the solvent, for example the dibutyl phthalate, is
separated off as a liquid stream at the side offtake.
Suitable vaporizers are fouling-insensitive and easy-to-clean heat
exchangers, for example falling film evaporators or thin film
evaporators, forced-circulation depressurization evaporators or
stirred vessels provided with, for example, an anchor stirrer going
around the wall.
Owing to the possibility of solid maleic acid or fumaric acid being
formed in the condenser at the top due to an increase in the
concentration, the condenser at the top should be designed so as to
be insensitive to formation of solids. A quench circuit or a
shell-and-tube heat exchanger through which liquid trickles are
useful for this purpose. The column should also be equipped with
solids-insensitive internals, for example dual flow trays or sheet
metal packing.
Thus, the low-boiling fraction is taken off continuously at the top
of a distillation column, the high-boiling fraction is taken off
continuously from the bottom of the column and the purified solvent
is taken off continuously in gaseous or liquid form via the side
offtake of the column in this first embodiment of the process of
the present invention.
A second embodiment of the process of the present invention is
likewise carried out continuously. In contrast to the first
embodiment, the tar-like high boilers are here separated off in an
upstream evaporator. The distillate is subsequently passed to a
distillation to separate off the low-boiling acidic constituents.
Here, as a difference from the first continuous embodiment, the
solvent, for example dibutyl phthalate, is taken off at the bottom
of the column. The evaporation in the preliminary evaporator and
the distillation are advantageously carried out under a reduced
pressure of, for example, from 0.1 to 100 mbar, preferably from 1
to 20 mbar, particularly preferably from 2 to 15 mbar. The
separation generally requires from about 1 to 60, in particular
from about 10 to 50, theoretical plates, preferably from about 9 to
37, particularly preferably from 12 to 27, theoretical plates.
Suitable upstream evaporators are fouling-insensitive and
easy-to-clean heat exchangers, for example falling film evaporators
or thin film evaporators, forced-circulation depressurization
evaporators or stirred vessels provided with, advantageously, an
anchor stirrer going around the wall. Although precipitation of
solids is not to be expected to the same degree as in the
preliminary evaporator, this phenomenon does also have to be taken
into account in the case of the distillation column. Owing to the
possibility of solid maleic acid or fumaric acid being formed in
the condensor at the top due to an increase in the concentration,
the condenser at the top should be designed so as to be insensitive
to formation of solids. A quench circuit or a shell-and-tube heat
exchanger through which liquid trickles are useful for this
purpose.
Thus, the low-boiling fraction and the fraction consisting
essentially of purified solvent are distilled off continuously from
the high-boiling fraction, and the low-boiling fraction is
separated off from the distillate via the top and the fraction
consisting essentially of purified solvent is separated off at the
bottom in the second embodiment of the process of the present
invention which has just been described.
A third embodiment is carried out batchwise. Here, a column is
superposed on a stirred vessel provided with, preferably, an anchor
stirrer going around the wall. The contaminated solvent is placed
in the stirred vessel. Two fractions are distilled off under
reduced pressure, firstly a low-boiling fraction comprising maleic
anhydride and maleic acid, fumaric acid, phthalic anhydride and
solvent, for example dibutyl phthalate. As main fraction, the
solvent, for example dibutyl phthtalate, goes over at the top.
After this concentration by distillation, a mixture of high boilers
and solvent, for example dibutyl phthalate, can be taken from the
stirred vessel. The discontinuous fractionation can be achieved,
for example, by reducing the pressure and/or increasing the
temperature. To keep the throughput through the column uniform, the
distillate can be taken off at one point, subjected to intermediate
storage and fed back into the column.
The fourth embodiment of the process of the present invention is
carried out semicontinuously. In contrast to variant 3, feed is
introduced into the stirred vessel during distillation. The
abovementioned low boilers go over at the top, while the solvent,
for example dibutyl phthtalate, is taken off via the side offtake.
No bottom product is taken off during the distillation and
accumulates during operation. As soon as a maximum level of liquid
phase is exceeded, the inlet valve is closed and the contents of
the stirred vessel are drained after breaking the vacuum. However,
continuous operation is in principle also possible by taking off
the bottom product continually or at intervals.
The process of the present invention is preferably carried out
using a solvent which has a boiling point which is higher than the
boiling points of the low boilers maleic acid, fumaric acid and
phthalic anhydride. Preference is given to using a solvent which
has a boiling point which is 5.degree. Kelvin higher, preferably
10.degree. Kelvin higher, most preferably 30.degree. Kelvin higher,
than the boiling points of the low boilers maleic acid, fumaric
acid and phthalic anhydride, so that these components can be
separated off as low boilers. Finally, the boiling point of the
solvent should be sufficiently low for it to be able to be
separated off from the tar-like polymers without decomposition
under an industrially feasible vacuum.
Possible solvents are dialkyl phthalates, for example those having
from 2 to 8 carbon atoms in each alkyl chain. Examples which may be
mentioned are dimethyl phthalate, diethyl phthalate, dipropyl
phthalate, diisopropyl phthalate, dibutyl phthalate, diisobutyl
phthalate, dimethyl dihydrophthalate, diethyl dihydrophthalate,
dipropyl dihydrophthalate, diisopropyl dihydrophthalate, dibutyl
dihydrophthalate, diisobutyl dihydrophthalate, dimethyl
tetrahydrophthalate, diethyl tetrahydrophthalate, dipropyl
tetrahydrophthalate, diisopropyl tetrahydrophthalate, dibutyl
tetrahydrophthalate and diisobutyl tetrahydrophthalate.
However, it is also possible to use monoalkyl phthalate compounds,
preferably those having from 2 to 8 carbon atoms in the alkyl
chain. Examples are monomethyl phthalate, monoethyl phthalate,
monopropyl phthalate, monoisopropyl phthalate, monobutyl phthalate,
monoisobutyl phthalate, monomethyl dihydrophthalate, monoethyl
dihydrophthalate, monopropyl dihydrophthalate, monoisopropyl
dihydrophthalate, monobutyl dihydrophthalate, monoisobutyl
dihydrophthalate, monomethyl tetrahydrophthalate, monoethyl
tetrahydrophthalate, monopropyl tetrahydrophthalate, monoisopropyl
tetrahydrophthalate, monobutyl tetrahydrophthalate and monoisobutyl
tetrahydrophthalate. Further suitable solvents are, for example,
dimethylbenzophenone, dichlorophenyl oxide, hexahydrophthalates and
monoalkyl-substituted succinic acids having from 12 to 16 carbon
atoms. A representative selection of solvents is described in WO
96/29323, which is hereby expressly incorporated by reference.
To achieve successful purification of the solvent, it is generally
sufficient for a substream of the circulating solvent to be
discharged. In general, this substream comprises from about 0.2 to
1% by weight, preferably from 0.3 to 0.7% by weight, of the total
amount of solvent.
The term "essentially" used here in the context of the fraction
consisting "essentially" of purified solvent means that the
fraction consists of solvent which has been purified to a
substantial extent. The purity of the solvent which has been
purified in this way can vary. It should be such that no
appreciable accumulation of acidic constituents takes place in the
solvent present in the absorption circuit during operation, since
otherwise there is a risk of the yield of MA decreasing. The purity
of the solvent purified in the substream is advantageously at least
95% by weight or higher, preferably at least 97% by weight,
particularly preferably at least 99% by weight.
When monoalkyl or dialkyl phthalates are used, the purity is
preferably at least 97% by weight, particularly preferably at least
99% by weight.
The invention is illustrated by the following examples.
EXAMPLE 1
10.0 kg/h of a stream composed of 0.5% by weight of MA, 1.0%, by
weight of maleic acid, 1.0% by weight of fumaric acid, 0.4% by
weight of tar-like high boilers, 1.0% by weight of phthalic
anhydride and 96.1% by weight of dibutyl phthalate were fed into a
column having 20 theoretical plates at theoretical plate no. 10
(counted from the top of the column). At a pressure of 10 mbar at
the top, a stream of 0.8 kg/h was taken off at the top. About 4
kg/h of runback were returned to the top of the column. The product
obtained at the top was composed of 6.3% by weight of MA, 56% by
weight of dibutyl phthalate and 12.5% by weight each of maleic acid
and fumaric acid, together with phthalic anhydride (PA). At the
bottom, 0.1 kg/h of a mixture consisting of 73% by weight of
dibutyl phthalate and 27% by weight of high boilers was taken off
at 228.degree. C. At the side offtake, which was located at the
seventeenth (counted from the top) theoretical plate, 9.1 kg/h of
dibutyl phthalate having a purity of 99.8% were obtained. The
recovery of dibutyl phthalate was 95%.
EXAMPLE 2
100 kg of a mixture composed of 0.5% by weight of MA, 1.0% by
weight of maleic acid, 1.0% by weight of fumaric acid, 0.4% by
weight of tar-like high boilers, 1.0% by weight of phthalic
anhydride (PA) and 96.1% by weight of dibutyl phthalate were placed
in a stirred vessel fitted with a superposed column having 20
theoretical plates. The apparatus was subsequently evacuated to 10
mbar. By means of a swivelling funnel, a substream of the
distillate was returned to the column. At a reflux ratio of 8, 10
kg of a first fraction were firstly taken off. During distillation,
the temperature at the top increased from 103.degree. C. to
190.degree. C. The distillate was collected in a vessel and had the
following composition: MA and maleic acid: 13% by weight; fumaric
acid: 19% by weight; phthalic anhydride: 10% by weight; dibutyl
phthalate: 68% by weight. The reflux ratio was subsequently changed
to 2 and 86 kg of dibutyl phthalate having a purity of 99.6% were
taken off at 190.degree. C. 4 kg of dibutyl phthalate containing
about 10% by weight of high boilers remained in the stirred vessel.
The recovery of dibutyl phthalate was 86%.
* * * * *